The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 1c2361f667f3648855ceae25f1332c18413fdb9f Mon Sep 17 00:00:00 2001 From: Sean Christopherson seanjc@google.com Date: Wed, 2 Feb 2022 00:49:44 +0000 Subject: [PATCH] KVM: x86: Use __try_cmpxchg_user() to emulate atomic accesses

Use the recently introduce __try_cmpxchg_user() to emulate atomic guest accesses via the associated userspace address instead of mapping the backing pfn into kernel address space. Using kvm_vcpu_map() is unsafe as it does not coordinate with KVM's mmu_notifier to ensure the hva=>pfn translation isn't changed/unmapped in the memremap() path, i.e. when there's no struct page and thus no elevated refcount.

Fixes: 42e35f8072c3 ("KVM/X86: Use kvm_vcpu_map in emulator_cmpxchg_emulated") Cc: stable@vger.kernel.org Signed-off-by: Sean Christopherson seanjc@google.com Message-Id: 20220202004945.2540433-5-seanjc@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index fb8153d584c8..e5b0dd24b200 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -7257,15 +7257,8 @@ static int emulator_write_emulated(struct x86_emulate_ctxt *ctxt, exception, &write_emultor); }

-#define CMPXCHG_TYPE(t, ptr, old, new) \ - (cmpxchg((t *)(ptr), *(t *)(old), *(t *)(new)) == *(t *)(old)) - -#ifdef CONFIG_X86_64 -# define CMPXCHG64(ptr, old, new) CMPXCHG_TYPE(u64, ptr, old, new) -#else -# define CMPXCHG64(ptr, old, new) \ - (cmpxchg64((u64 *)(ptr), *(u64 *)(old), *(u64 *)(new)) == *(u64 *)(old)) -#endif +#define emulator_try_cmpxchg_user(t, ptr, old, new) \ + (__try_cmpxchg_user((t __user *)(ptr), (t *)(old), *(t *)(new), efault ## t))

static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt, unsigned long addr, @@ -7274,12 +7267,11 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt, unsigned int bytes, struct x86_exception *exception) { - struct kvm_host_map map; struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt); u64 page_line_mask; + unsigned long hva; gpa_t gpa; - char *kaddr; - bool exchanged; + int r;

/* guests cmpxchg8b have to be emulated atomically */ if (bytes > 8 || (bytes & (bytes - 1))) @@ -7303,31 +7295,32 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt, if (((gpa + bytes - 1) & page_line_mask) != (gpa & page_line_mask)) goto emul_write;

- if (kvm_vcpu_map(vcpu, gpa_to_gfn(gpa), &map)) + hva = kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa)); + if (kvm_is_error_hva(addr)) goto emul_write;

- kaddr = map.hva + offset_in_page(gpa); + hva += offset_in_page(gpa);

switch (bytes) { case 1: - exchanged = CMPXCHG_TYPE(u8, kaddr, old, new); + r = emulator_try_cmpxchg_user(u8, hva, old, new); break; case 2: - exchanged = CMPXCHG_TYPE(u16, kaddr, old, new); + r = emulator_try_cmpxchg_user(u16, hva, old, new); break; case 4: - exchanged = CMPXCHG_TYPE(u32, kaddr, old, new); + r = emulator_try_cmpxchg_user(u32, hva, old, new); break; case 8: - exchanged = CMPXCHG64(kaddr, old, new); + r = emulator_try_cmpxchg_user(u64, hva, old, new); break; default: BUG(); }

- kvm_vcpu_unmap(vcpu, &map, true); - - if (!exchanged) + if (r < 0) + goto emul_write; + if (r) return X86EMUL_CMPXCHG_FAILED;

kvm_page_track_write(vcpu, gpa, new, bytes);